To those wondering, I ended up going with the td100x mixvent to great success. Was using Satori from Mandala seeds. Sure it's a $100 fan, but any decent grower can make up that value in one run imo.
I've since had to completely dump my old gear (better safe than sorry when events pop up). I was researching on getting a new mixvent and stumbled on my old thread.
My new growroom setup: (no more growbox)
garage closet appx 40-50 sq ft
box fan blowing over the lamp (using a basic reflector hood)
temps run ~72F ambient and ~82 12" under the lamp
The plan is to build another cooltube, this time with no intake. The exhaust end of the fan will run to a diy carbon scrubber like the one from this thread on another site